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Chapter 4. Plate Tectonics

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Chapter 4. Plate Tectonics Physical Geology, First University of Saskatchewan Edition is used under a CC BY-NC-SA 4.0 International License Read this book online at http://openpress.usask.ca/physicalgeology/ Chapter 4. Plate Tectonics Adapted by Karla Panchuk from Physical Geology by Steven Earle Figure 4.1 Iceland is known for its volcanoes, which are present because Iceland is located on the Mid-Atlantic Ridge, where the Atlantic Ocean is spreading apart and new crust is forming. In fact, Iceland exists because that volcanic activity has built up the island from the ocean floor. Iceland is cut by rift zones (white lines on the map at left) where the island is splitting apart along with the rest of the Atlantic Ocean. Rift zones are marked by belts of young volcanic rocks (dark green). You can stand on a rift zone if you visit Thingvellir National Park (right). Rifting has produced a valley where the crust has settled downward. The margins of the North American and Eurasian tectonic plates are visible as ridges on either side of the valley. The photographer was standing on a ridge on the North American side. Source: Karla Panchuk (2018) CC BY-SA 4.0. Photo: Ruth Hartnup (2005) CC BY 2.0. See Appendix C for more attributions. Learning Objectives After reading this chapter and answering the review questions at the end, you should be able to: • Discuss the early evidence for continental drift, and Alfred Wegener’s role in promoting this theory. • Describe other models that were used early in the 20th century to understand global geological features. • Summarize the geological advances that provided the basis for understanding the mechanisms of plate tectonics, and the evidence that plates and are constantly being created and destroyed. • Describe the seven major plates, including their size, motion, and the types of boundaries between them. • Describe the geological processes that take place at divergent and convergent plate boundaries, and explain why transform faults exist • Explain how supercontinents form and how they break apart. • Explain why tectonic plates move. Plate tectonics is the model or theory that we use to understand how our planet works: it explains the origins of continents and oceans, the origins of folded rocks and mountain ranges, the presence of different kinds of rocks, the causes and locations of earthquakes and volcanoes, and changes in the positions of continents over time. So... everything! Chapter 4. Plate Tectonics 1 The theory of plate tectonics was proposed to the geological community more than 100 years ago, so it may come as a surprise that an idea underpinning the study of Earth today did not become an accepted part of geology until the 1960s. It took many decades for this theory to become accepted for two main reasons. First, it was a radically different perspective on how Earth worked, and geologists were reluctant to entertain an idea that seemed preposterous in the context of the science of the day. The evidence and understanding of Earth that would have supported plate tectonic theory simply didn’t exist until the mid- twentieth century. Second, their opinion was affected by their view of the main proponent, Alfred Wegener. Wegener was not trained as a geologist, so he lacked credibility in the eyes of the geological community. Alfred Wegener was also German, whereas the geological establishment was centred in Britain and the United States- and Britain and the United States were at war with Germany in the first part of the 20th century. In summary, plate tectonics was an idea too far ahead of its time, and delivered by the wrong messenger. 4.1 Alfred Wegener’s Arguments for Plate Tectonics Alfred Wegener (1880-1930; Figure 4.2) earned a PhD in astronomy at the University of Berlin in 1904, but had a keen interest in geophysics and meteorology, and focused on meteorology for much of his academic career. In 1911 Wegener happened upon a scientific publication that described matching Permian-aged terrestrial fossils in various parts of South America, Africa, India, Antarctica, and Australia. He concluded that because these organisms could not have crossed the oceans to get from one continent to the next, the continents must have been joined in the past, permitting the animals to move from one to the other (Figure 4.3). Wegener envisioned a supercontinent made up of all the present day continents, and named it Pangea Figure 4.2 Alfred Wegener during a 1912-1913 (meaning “all land”). He described the motion of the expedition to Greenland. Source: Alfred Wegener continents reconfiguring themselves as continental drift. Institute (2008) Public Domain Figure 4.3 The distribution of several Permian terrestrial fossils that are present in various parts of continents now separated by oceans. During the Permian, the supercontinent Pangea included the supercontinent Gondwana, shown here, along with North America and Eurasia. Source: J.M. Watson, USGS (1999) Public Domain. Chapter 4. Plate Tectonics 2 Wegener pursued his idea with determination, combing libraries, consulting with colleagues, and making observations in an effort to find evidence in support of it. He relied heavily on matching geological patterns across oceans, such as sedimentary strata in South America matching those in Africa, North American coalfields matching those in Europe, the mountains of Atlantic Canada matching those of northern Britain—both in structure and rock type—and comparisons of rocks in the Canadian Arctic with those of Greenland (Figure 4.4). Wegener also called upon evidence for the Carboniferous and Permian (~300 Ma) Karoo Glaciation from South America, Africa, India, Antarctica, and Australia (Figure 4.5). He argued that this could only have happened if these continents were once all connected as a single supercontinent. He also cited evidence (based on his own astronomical observations) that showed that the continents were moving with respect to each other, and determined a separation rate between Greenland and Scandinavia of 11 m per year, although he admitted that the measurements were not accurate. (The separation rate is actually about 2.5 cm per year.) Wegener first published his ideas in 1912 in a short book called Die Entstehung der Kontinente (The Origin of Figure 4.4 Diagram from Alfred Wegener's book Die Continents), and then in 1915 in Die Entstehung der Entstehung der Kontinente und Ozeane comparing rock types Kontinente und Ozeane (The Origin of Continents and on Canadian Arctic Islands and Greenland. Source: Karla Oceans). He revised this book several times up to 1929. It Panchuk (2018) CC BY 4.0. See Appendix C for more was translated into French, English, Spanish, and Russian in attributions. 1924. The main criticism of Wegener's idea was that he could not explain how continents could move. Remember that, as far as anyone was concerned, Earth's crust was continuous, not broken into plates. Thus, any mechanism Wegener could think of would have to fit with that model of Earth's structure. Geologists at the time were aware that continents were made of different rocks than the ocean crust, and that the material making up the continents was less dense, so Wegener proposed that the continents were like icebergs floating on the heavier ocean crust. He suggested that the Figure 4.5 Carboniferous and Permian Karoo Glaciation in continents were moved by the effect of Earth's rotation the southern hemisphere. Paleogeographic reconstruction for pushing objects toward the equator, and by the lunar and 306 million years ago. Source: Cropped from C. R. Scotese, solar tidal forces, which tend to push objects toward the PALEOMAP Project (www.scotese.com) view source. See west. However, it was quickly shown that these forces were Appendix C for terms of use. far too weak to move continents, and without any reasonable mechanism to make it work, Wegener’s theory was quickly dismissed by most geologists of the day. Alfred Wegener died in Greenland in 1930 while carrying out studies related to glaciation and climate. At the time of his death, his ideas were tentatively accepted by a small minority of geologists, and firmly rejected by most. But within a few decades that was all to change. 4.2 Global Geological Models of the Early 20th Century The untimely death of Alfred Wegener did not solve any problems for those who opposed his ideas, because they still had some inconvenient geological truths to deal with. One of those was explaining the distribution of terrestrial species across five continents that are currently separated by hundreds or thousands of kilometres of ocean water, and another was explaining the origin of extensive fold-belt mountains, such as the Appalachians, the Alps, the Himalayas, and the Canadian Rockies. Chapter 4. Plate Tectonics 3 Before we continue, it is important to know what was generally believed about global geology before plate tectonics. At the beginning of the 20th century, geologists had a good understanding of how most rocks were formed and understood their relative ages through interpretation of fossils, but there was considerable controversy regarding the origin of mountain chains, especially fold-belt mountains. At the end of the 19th century, one of the prevailing views on the origin of mountains was the theory of contractionism — the idea that since Earth is slowly cooling, it must also be shrinking. In this scenario, mountain ranges had formed like the wrinkles on a dried-up apple. Oceans formed above parts of former continents that had settled downward and become submerged. While this hypothesis helped to address the dilemma of the terrestrial fossils by explaining how continents once connected could now be separated by oceans, it came with its own set of problems. One problem was that Earth wasn't cooling fast enough to create the necessary amount of shrinking.
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